Hardware Reference
In-Depth Information
does not have enough head room for standard registered DIMMs. Some vendors sell only
this type of DIMM for particular systems.
The important thing to note is that you can use only the type of module your motherboard
(or chipset) is designed to support. For most, that is standard unbuffered modules or, in
some cases, registered modules.
SIMM Details
Some of the factors to keep in mind when installing memory into a system that uses
SIMMs include:
• How different types of SIMMs identify themselves (presence detect pins)
• Differences between generic and IBM-specific SIMMs
• Compatibility issues when mixing gold and tin modules and contacts.
SDR DIMM Details
SDRDIMMsuseacompletelydifferenttypeofpresencedetectthanSIMMs,called
serial
presence detect
(SPD). It consists of a small EEPROM or flash memory chip on the
DIMM that contains specially formatted data indicating the DIMM's features. This serial
data can be read via the serial data pins on the DIMM, and it enables the motherboard to
autoconfigure to the exact type of DIMM installed.
DIMMs can come in several varieties, including unbuffered and buffered as well as 3.3V
and 5V. Buffered DIMMs have additional buffer chips on them to interface to the mother-
board. Unfortunately, these buffer chips slow down the DIMM and are not effective at
higherspeeds.Forthisreason,mostPCsystems(thosethatdonotuseregisteredDIMMs)
use unbuffered DIMMs. The voltage is simple—DIMM designs for PCs are almost uni-
versally 3.3V. If you install a 5V DIMM in a 3.3V socket, it would be damaged, but key-
ing in the socket and on the DIMM prevents that.
Modern PC systems use only unbuffered 3.3V DIMMs. Apple and other non-PC systems
canusethebuffered5Vversions.Fortunately,thekeynotchesalongtheconnectoredgeof
a DIMM are spaced differently for buffered/unbuffered and 3.3V/5V DIMMs, as shown
in
Figure 6.11
.
This prevents inserting a DIMM of the wrong type into a given socket.
Figure 6.11
The 168-pin DRAM DIMM notch key definitions.
